Traditionally, metal containers destined for the retail market have been either "two piece" or "three piece" types. Two piece cans, which are typically aluminum, have traditionally been used as beverage containers in which strength requirements are low. Such cans are formed by joining a drawn and ironed cylindrical body to a circular lid. Two piece cans are decorated individually in their cylindrical configuration using lithographic printing presses such as that disclosed in U.S. Pat. Nos. 3,223,028 (Brigham), 3,227,070 (Brigham et al.), 3,766,851 (Sirvet et al.) and 4,138,941 (McMillin et al.). In such presses, a number of inking units are arranged around a central blanket cylinder. Each inking unit transfers the portion of the image to be printed in a single color to the blanket cylinder that then transfers the entire image to the can surface in a single pass. After printing, the cans are cured by baking in an oven. In order to prevent the colors from running together on the blanket, each color must be separated from the adjacent color by a small blank space, referred to as a "trap line."
Three piece cans have been used for many types of food stuffs and also for aerosol containers in which strength is an important requirement. Three piece cans are generally made of steel and are formed by attaching a circular top and a circular bottom to a cylindrical body portion. The body portion is formed by bending a flat rectangular plate, referred to as a body blank, into a cylinder and welding the overlapping longitudinal edges of the body blank to form a joint. The decorations for a three piece can are printed on multi-body blank sheets while still in a flat configuration.
Heretofore, it had been thought that body blanks for three piece cans could only be efficiently printed in a high speed production line by printing a substantial number of the container images in a rectangular array on a large flat metal sheet. After printing, the large sheet was cut into individual body blanks, with each body blank being used to form one can body. It was thought to be economically infeasible to separately print each can body individually in a high speed production environment.
In the traditional three piece can printing approach, the large flat sheet was printed by passing it through a two color lithographic printing press. The press employed two printing plate cylinders, each of which contained the portion of the image to be printed in a single color ink. The inks from both printing plate cylinders were applied to a common blanket cylinder that then applied the two color images to the sheet in a single pass. A number of the images to be printed were arranged side by side on the printing plates so that a row of container images was printed by each rotation of the blanket cylinder. The partially printed sheet was then baked in an oven to cure the inks.
After printing in the first two colors, the printing plates were changed to those containing the portions of the images to be printed in two different colors and the press was supplied with the new color inks. The sheet was then fed through the press in a second pass and re-baked in the oven to cure the newly applied ink. This process was repeated until all the required colors were applied to the sheet. The end result was a large sheet containing a number of can decorations in a rectangular array of rows and columns. After printing, the sheet was cut longitudinally and transversely using a slitter so as to form individual body blanks that were then formed into cylindrical container bodies. Since the inks were cured between each pass it was not necessary to form the aesthetically unappealing trap lines, required in the aforementioned two piece can printing process, to prevent the inks from running together.
Unfortunately, the traditional approach to body blank printing suffers from a variety of drawbacks. First, it is difficult to maintain uniform pressure of the blanket cylinder across such a wide sheet. As a result, the images on the body blanks cut from the center portion of the sheet are often lighter than those cut from the edge portions, causing a lack of uniformity in product appearance when the cans are displayed on a supermarket shelf.
Second, since many cans require printing in eight colors, four separate passes through the press are required. This results in considerable downtime to change the printing plates and clean up the previously used inks.
Third, since the registration of the sheet with the printing plates must be reset for each pass, the image printed in each pass may not be in precise registration with the previously printed images. This problem creates significant inefficiencies when the out-of-registration condition is discovered after the third or fourth pass since the entire sheet must then be scrapped and the process begun again from scratch.
Accordingly, it would be desirable to provide an efficient high speed method of printing body blanks in which all of the colors were applied in a single pass so that the body blanks are registered to the printing press only once. Such a method should allow for the printing of individual body blanks so that (i) uniform pressure of the blanket cylinder against the body blanks can be maintained, (ii) printing problems can be rapidly detected before unnecessary additional printing was performed, and (iii) if misprinting occurs, only the improperly printed body blanks need be scrapped.
It is known to print paper sheets in a four color printing press utilizing a central rotating impression cylinder about which a number of inking units, each containing a different color ink, are disposed - see, U.S. Pat. No. 4,936,211 (Pensavecchia). Such presses utilize a cam operated clamp to grip the sheets of paper to the impression cylinder which carries the sheet sequentially under each of the inking units. As in the case of the aforementioned press for printing body blanks, it had been though most efficient for such paper presses to print a number of document pages onto a large sheet of paper that is subsequently cut into individual pages. Hence, such paper presses do not solve the problem discussed above of a lack of uniformity in printing across the width of a wide sheet. However, such presses allow multiple colors to be printed in a single pass around the impression cylinder without the need to repeatedly re-register the sheet.
Unfortunately, paper sheet printing presses are not suitable for printing metal plates, such as container body blanks, for several reasons. First, there is no provision in such presses for curing each layer of ink before the sheet passes under the next inking unit. This presents no problem in printing paper sheets since no curing is required to prevent smearing. However, ink applied to a metal substrate must be cured prior to passing it under the next inking unit to prevent smearing and mixing of the inks. It had always been thought that this problem precluded the use of such rotary presses in a high speed metal printing operation because it is infeasible to pass the metal through a curing oven between each inking unit.
A second problem arises with respect to clamping metal body blanks to the impression cylinder. Due to their extreme flexibility, the paper sheets can be readily made to lie flat against the impression cylinder. Consequently, such presses utilize a single clamp to secure each paper sheet to the impression cylinder. Metal body blanks, however, are stiffer and can not be stably secured to the impression cylinder as readily.
Third, such presses utilize rollers to drive the paper sheets around curved guide rollers to bring the sheets into engagement with the clamps. Unfortunately, metal plates are too stiff to be directed to the impression cylinder in this manner.
Fourth, it would be difficult to incorporate more that four inking units into such presses so that the number of colors that can be applied in a single pass is limited.
Accordingly, it would be desirable to provide an efficient high speed press for separately printing individual body blanks in which all of the colors were applied in a single pass of an impression cylinder and that over came the problems of paper presses by (i) developing a method for curing the ink as the body blanks passed between inking units, (ii) developing a method of securely clamping the body blank to the impression cylinder, and (iii) developing a method of properly directing the body blanks to the impression cylinder so that they can be registered and securely clamped into position for all overlapping decorations, thereby allowing ink depositions to be accurately placed.